Various insecticides for agricultural uses are known to be highly toxic to most species of insects and mites. However, many of these same materials (e.g., phosphorothioates and phosphorodithioates) are poisonous to higher animals including man, and there are many reported cases of injury and even death to workers in areas sprayed with poisonous insecticides. Lethal doses can be inhaled or absorbed through the skin.
In recent developments, many of the poisonous insecticides and fungicides have been encapsulated with polymers, especially polyamides. For example, methyl parathion has been encapsulated in a mixed polyamide-polyurea polymeric skin and the resulting capsules are commercially available as aqueous dispersions (e.g., Penncap M, by Pennwalt Corporation). Encapsulation is extremely significant from the standpoint of safety in the handling of these materials in that the capsules can be safely dispersed in water and applied by spray apparatus in the form of aqueous dispersions. Unfortunately, most of the insecticides which would benefit from encapsulation are also heavier than the surrounding aqueous dispersion media and, therefore, the capsules tend to settle out quickly, i.e., the dispersions tend to "break" soon after formulation to form two distinct phases or layers. The upper layer is generally clear and is believed to consist essentially of portions of the carrier liquid. The lower layer is generally opaque and constitutes the original dispersion minus some of the carrier liquid which has separated to form the upper clear liquid level. Finally, after standing, portions of the insecticide capsules generally separate from the lower dispersion phase and settle on the bottom of the container forming a third phase. Once settled, the capsules in this third layer tend to adhere to one another and cannot be easily redispersed in the field because only simple mixing equipment is generally available to most users under these conditions. This lack of redispersibility prevents these safer encapsulated forms of the insecticides from being uniformly applied by agricultural spray applicators. This is a serious disadvantage because in one area of a field, for example, the amount of material applied may be insufficient to give good control over the insect pest, whereas in other areas of the same field, the amount applied may be overdone to the point that toxicity problems arise.
What is needed to reduce present difficulties and to permit wide use of the safer encapsulated insecticides is a means for forming the insecticide capsules into essentially stable aqueous dispersions thereby minimizing amounts of material which must be redispersed. But, recognizing that even the best present dispersions tend to undergo some separation upon standing for prolonged periods, a dispersing agent is needed which not only forms a relatively stable dispersion but also interacts with the capsules to minimize packing in sedimentary layers resulting from settling, and permit the capsules to be easily redispersed using relatively simple mixing equipment. Surprisingly, these objects and advantages are obtained by my invention as described below.